When storing filling goods, such as food, beverages, or the like, in cans, there exists the danger that the respective stored goods will, for instance in view of acidic ingredients, attack the metallic container, and inversely it may happen that components of the metallic container are absorbed by the stored goods so that these goods will be spoiled instead of preserved. In order to prevent such reactions, the inside of the cans may be provided with a protective varnish coat as insulating layer between the metallic wall of the container and the respective goods. In order to realize an economical manufacture of such cans, the varnish coat or layer must dry quickly so that the cans can rapidly be conveyed to the next processing station.
For the same reason a quick drying of outside varnish layers and/or paints of printed-on pictures to be applied to the cans is desired.
The varnishing of the inside of cans is presently used to a great extent, especially in connection with the manufacture of cans for beverages. In this connection, the cans are varnished in inside varnishing automats by the spraying method and are then by means of one or more expensive transporting or conveying systems conveyed to a drying furnace.
With so-called three-sectional cans, the bodies of which are made from sheet metal blanks, the outside varnishing and printing is effected already on the sheet metal. To this end, blanks already printed upon in color are used. With so-called two-sectional cans, the bodies of the cans which may be, for instance, deep drawn and stretched, are printed upon by rollers on their outer circumference and are then conveyed to drying furnaces.
Commercially customary drying furnaces for this purpose work in conformity with the convection or heat radiation principle and are heated electrically or by gas. The heat radiating surfaces and the heating gases are considerably hotter than the desired end temperature of the cans. In order to assure that the cans are not heated up to a temperature higher than the desired end temperature, it is necessary that the cans pass through the drying furnace in a certain period of time. If this time cannot be maintained, for instance, due to irregularities in the operation of the furnace, the varnish will burn and the respective furnace charge must be discarded as rejects. Inasmuch as the temperature of the cans during the passage of the cans through the furnace increases from room temperature to the desired maximum temperature, which maximum temperature corresponds substantially to the optimum temperature for hardening the varnish, it will be appreciated that during a considerable portion of the passage of the cans through the furnace the temperature of the cans is below the temperature which is the optimum temperature for hardening the varnish. This results in a poor exploitation of the drying installation. Inasmuch as the most favorable duration of the passage of the cans through the furnace can be estimated only approximately, and since the actually most favorable time can only be determined by tests made during the operation of the furnace, the output of the drying installation cannot easily be adapted to the production output of the other machines. The output of the drying installation should, however, at least equal the production output of the preceding machines, and it must not be higher than the production output of the succeeding machines if work stoppages or idling times are to be avoided.
It is, therefore, an object of the present invention to provide a method of and device for drying varnishes and/or printing dyes or paints applied to the inside or outside of the can bodies, which will overcome the above referred to drawbacks and will in particular assure that the varnished and/or printed upon cans will not be heated beyond a certain maximum temperature which maximum temperature should substantially equal the optimum temperature for hardening the varnish or paint.
It is another object of the present invention to provide a method and device as set forth above according to which the hardening process can be carried out in a very short and economical time so as to assure an optimum exploitation of the installation.
These and other objects and advantages of the invention will appear more clearly from the following specification in connection with the accompanying drawing, in which: